This invention relates generally to microdevices, including micromachined capacitive pressure sensors and, in particular, to the prevention, reduction and elimination of outgassing and trapped gases in such devices.
Micromachined sensors have been under intense development during the last two decades, and many devices have been developed and commercially produced. Silicon-based pressure sensors, in particular, are being sold worldwide in very large volumes, and are being used in a wide variety of application areas, including automotive and industrial processing.
As part of the fabrication process for the development of vacuum referenced micromachined transducers, attachment of silicon to a substrate, (for example, bonding silicon to glass substrates) in vacuum is required. Various attachment procedures include, but are not limited to, anodic bonding, fusion bonding, thermal bonding, eutectic bonding, glass frit bonding (thermal compression bonding), adhesive bonding and chemical bonding.
Creating ultra high vacuum levels inside micromachined cavities is, however, a problem. For example, even though anodic bonding can be done in very high vacuum levels (for example 10xe2x88x926 torr), the pressure inside the vacuum bonded cavities is generally much higher (up to a few orders of magnitude higher). This suggests that there might be at least two residual gas sources which contribute to this lack of ultra high vacuum sealing of transducers. One source is gas generation during the bonding process, and the other source is gas desorption from the inner surface of the sealed cavity. Therefore, in order to create high vacuum in cavities, these gas sources must be eliminated.
This invention is directed to both preventing outgassing and the elimination of gases created during attachment inside vacuum cavities.
In general terms, this invention is directed toward the prevention of outgassing and/or the elimination of gases created during attachment inside micromachined vacuum cavities. In the preferred embodiment, such problems are solved in a manner which is mass manufacturable.
Specifically, the invention resides in two broad approaches to the problem of sealing ultrahigh vacuum cavities, which can be applied separately or in combination. One method is to deposit a barrier layer within the cavity (for example, on an exposed surface of the substrate). Such a layer not only provides a barrier against gases diffusing out of the substrate, but is also chosen so as to not outgas by itself.
Another approach is to use a material which, instead of, or in addition to, acting as a barrier layer, acts as a getterer, such that it reacts with and traps unwanted gases.
Incorporation of a getterer according to the invention can be as straightforward as depositing a thin metal layer on the substrate, which reacts to remove the impurities, or can be more elaborate through the use of a non-evaporable getter in a separate cavity in gaseous communication with the cavity.
As discussed in the detailed description which follows, the invention is applicable to a wide range of micromachined devices and structures, including micromachined pressure sensors and other transducers, though the techniques are not limited to such applications. In the preferred embodiment, anodic bonding is used in conjunction with a glass substrate, with the understanding that various types of substrates and bonding/attachment procedures may alternatively be used.